Greece's goal of reducing its gargantuan debt received a fresh blow today when the EU statistics agency announced that the country's 2009 budget deficit was much worse than first thought.

Six months after Athens received €110bn (£93bn) in emergency loans from EU nations and the International Monetary Fund to prop up its near-bankrupt economy, Eurostat revealed that Greece's budget deficit reached 15.4% of GDP last year, substantially higher than its previous estimate of 13.6%.

In April, Eurostat had estimated the debt-to-GDP ratio would reach 115.1%. The revised data meant that Greece's debt ratio has eclipsed those of every other EU state, officials said. By the end of 2009, its debt is projected to account for 126.8% of GDP.

Greece's poor bookkeeping was blamed for the budget black holes.

As a team of visiting inspectors from the IMF, the European commission and the European Central Bank arrived in Athens, there was widespread acceptance that the new figures would throw out the fiscal and structural reform programme the socialist government has agreed to in return for the loans, the biggest bailout in history.

"We will face a profound strategic issue of how to repay €70bn-€80bn when redemption of the rescue loans comes," a senior government aide told the Guardian. "There will have to be some disguised rescheduling of the time frame in which we repay the money."

Prime minister George Papandreou's administration had previously insisted it would slash the deficit to 8.1% of GDP by the end of the year before lowering it to the permissible EU level of 3% in 2013 when the rescue programme expires.

In government for 13 months, the socialists have been heavily criticised by Greeks for their austerity policies. But in an interview at the weekend, Papandreou admitted for the first time that Athens may have to seek an extension of the repayment deadline. He also conceded that the revision could mean further cost-cutting.

The centre-left administration has already imposed tax hikes and wage and pension reductions that have triggered violent street protests. One well-placed insider said that, with an extra €7bn-€8bn needed by the end of the year, it was "very likely" the government would press ahead with plans to shut down parts of the bloated public sector.

Unionists backed by the Communist party have warned that further measures could prove the tipping point where popular opposition turns into explosive social unrest. Mass demonstrations and strikes are planned in the coming weeks.

Greece was compelled to accept the rescue package after it was effectively locked out of capital markets because of prohibitively high borrowing costs earlier in the year.

Since then, the country's public finances have been closely monitored by visiting inspectors from the European Commission, ECB and IMF. So far Athens has received two instalments of aid with a third expected later this month amounting to a total of €38bn. But the three organisations have made it clear that further handouts depend on the country staying the course and making fiscal progress.